Add a mechanism to try the database migration on a copy of the tables before running it on the "real" data

- kill replaceDB() - this function is unused and it's implementation obviously wrong
- add method annotation OC_DB_StatementWrapper::fetchAll
- remove duplicate code in Test_DBSchema and reuse OC_DB::tableExists
- remove unused variables

* Use "filesystem_cache_readonly" config setting, update comment in config.sample
* Use $this->cacheActive to cache config setting
* Add public Scanner::setCacheActive() to set $cacheActive programmatically